The problem of aerodynamic surfaces becoming iced is well known in the aviation industry. The term “icing” is used to designate more or less rapid formation of a deposit of ice on certain portions of an aircraft (leading edges of blades, propellers, wings, tail stabilizers and fins, windscreens, etc.). This ice forms in flight because an aerodynamic surface encounters droplets of water in the atmosphere that are supercooled. This supercooled state is a very precarious equilibrium state that can be broken by supplying a very small quantity of energy to the water droplet, e.g. in the form of a mechanical shock. The water then changes state and passes to the solid state. Thus, an airplane wing or a rotorcraft blade, for example, on passing through a zone of supercooled rain delivers enough energy to all of the water droplets it encounters for them to pass into the solid state. The aerodynamic surface then becomes covered in ice very quickly. This ice deposit weighs down the aircraft, sometimes to a very considerable extent, and also spoils the air flow by changing the shape of the aerodynamic surface, thus greatly degrades its performance.
This problem is often countered by fitting the aerodynamic surface with a heater mat, such a mat comprising a resistor element made of electrically-conductive fibers, e.g. carbon fibers, integrated in a composite substrate. When an electric current is passed therethrough, the Joule effect causes the resistor element to heat up the aerodynamic surface in which it is implanted so as to de-ice it or protect it against icing.
One of the difficult points with that technology lies with feeding the resistor element with electricity, and more particularly lies with the connection between at least one end of said resistor element and one or more electrical power supply wires coming from the aircraft.
Document FR 2 578 377 discloses the technique in present use, which consists in providing a deformable tubular sheath constituted by a metal knit. That tubular sheath serves to provide an electrical connection between a resistor element made of electrically-conductive fibers and an electrical power supply wire.
A first end of the tubular sheath is wound around the electrical power supply wire and is then soldered thereto. A second end thereof is then engaged around one end of the resistor element. The assembly as made in this way is then placed on a composite substrate and covered in one or more layers of fiberglass cloth in order to finish off draping the heater mat. The pressure applied during the operation of polymerizing the heater mat serves to anchor the metal knit of the tubular sheath in the electrically-conductive fibers of the resistor element.
Although that connection device gives satisfaction, it nevertheless presents characteristics that are poorly compatible with the requirements of industrial manufacturing.
Firstly, it requires the electrical power supply wire(s) to be connected to the resistor element at the time the heater mat is being made. When the heater mat is used for de-icing a blade of composite material for a rotorcraft, electrical power supply wires (often having a length that is greater than one meter) prevent the heater mat being installed on the blade while the blade is itself being molded. The electrical power supply wires are difficult to incorporate in the blade mold and they run the risk of leading to significant defects in molding.
Under such conditions, the present solution can be implemented only on an already-polymerized blade, during a specific bonding operation. The cycle time and manufacturing cost of the blade are then increased in penalizing manner.
Secondly, the high degree of flexibility and deformability of the tubular sheath lead to implementation difficulties, particularly for holding the assembly comprising the tubular sheath and the electrical power supply wire in position on the resistor element while the heater mat is being draped. Furthermore, the molding pressure exerted during polymerization of the heater mat can lead to misalignments and deformations of the electrical connections. These defects lead to poor quality electrical contact that can lead to abnormal levels of local heating.
Finally, the knitted structure of the tubular sheath leads to considerable electrical resistance in the longitudinal direction. This resistance thus greatly limits the magnitude of electric current that can be delivered to the resistor element. At the current levels required for good operation of the heater mat, this resistance can lead to excessive heating of the electrical connection zone.
A known variant of the prior art technique consists in soldering the electrical power supply wires not to an end of the tubular sheath but to the entire length thereof so as to allow a high current to pass. Nevertheless, that type of connection leads to considerable extra thickness in the electrical connection zone. Under such conditions, the aerodynamic surfaces need to be specially arranged inside the volume they define in order to be able to receive the device, and sometimes that can make it impossible to integrate the device on already-existing aerodynamic surfaces.
Furthermore, since the thickness of the connection needs to be minimized, it is necessary to spread out the strands of the electrical power supply wire(s) over the surface of the tubular sheath during tinplating. That operation is difficult and often leads to a surface that is not very uniform. Even when performed by experienced operators, manufacturing time is long and the final result is poorly reproducible, which naturally is not compatible with satisfactory industrialization of the device.